The catalytic oxidation of ammonia for the production of nitric acid is known (as Ostwald process). The oxidation of ammonia for large-size production of NO as the first product of the process is achieved on, for example, PtRh or PtRhPd alloy catalysts at a high temperature (usually 800 to 950° C.). Said alloy catalysts are piled up in the reactor in several layers one upon the other, usually in the form of knitted or woven gauzes. Typically, PtRh or PtRhPd wire which is, for example, 76 μm in diameter is used for producing said gauzes.
Since the oxidation of ammonia causes noble metal to be lost via the gas phase in the form of oxide, a system of recovery gauzes is usually arranged underneath the catalyst gauzes, said recovery gauzes being used to collect a part of the platinum. In a replacement reaction with the platinum, it is instead the lighter-weight and lower-priced palladium that passes over to the gas phase.
During the production of nitric acid, the NO that is initially developing at the catalyst gauzes is oxidized to the higher-valency NO2. After having been cooled down, the gas flow is directed to absorption towers, and the nitrogen monoxides are absorbed in water. The conversion to nitric acid is then achieved with additional oxygen.
Nitrogen and N2O result as by-products from the ammonia oxidation. In contrast to NO and NO2, N2O does not enter into any further reactions and, after having run through all process stages, is released into the atmosphere. When conventional catalyst gauzes and recovery gauzes are used, an amount of N2O ranging from 500 to 3000 ppm is released into the environment, unless the N2O is removed subsequently (EP 1 076 634 B1).
Some time ago, N2O was classified as a climatically harmful gas, since it both affects the ozone layer adversely and contributes to global heating. Since its potential for global heating exceeds that of CO2 by a factor of about 310, relatively low emissions of N2O are sufficient to contribute to global heating to a considerable degree. For that reason, efforts are made to reduce anthropogenic N2O emissions.
In other words, the content of N2O in the product gas of ammonia oxidation should be reduced. This can be achieved either by reducing the formation of N2O or by degrading it.
A number of catalysts have already been proposed for the decomposition of laughing gas. Applications and requirements vary depending on whether exhaust gas is purified, laughing gas from excessive anesthetic gas is destroyed, or ammonia is removed from industrial waste gases. The carrier material also has an influencing effect. For example, rhodium (oxide) on gamma-Al2O3 does not result in any reduction of N2O in the Ostwald process. On the other hand, gamma-Al2O3 is, for the most part and to advantage, used in the case of catalytic converters.
Systems with Pt, Rh, Ru and/or Ir have, for example, already been disclosed for catalytic converters. For example, JP 06142509 recommends a percentage by weight of Rh ranging from 0.3 to 2 on alpha-Al2O3 for temperatures ranging from 300 to 500° C. for the removal of N2O, Said document also mentions Ru and Ir as catalysts. These metals, in turn, do not have any effect in the Ostwald process.
JP 6142517 A1 describes catalysts with alpha-Al2O3 as carrier material, which comprise Rh or Ru and at least one of the oxides of Ti, Zr or Nb. The fact that Ru is not suited for the Ostwald process applies here as well. Furthermore, titanium and niobium oxides are not suitable either.
According to JP 2002253967 A1, Ru or Pd on SiO2 or Al2O3 are also used to destroy laughing gas that is excessive as anesthetic gas. Appropriate reactors are steel pipes which are packed with Al2O3 grains that are coated with noble metal and are operated at 150 to 550° C. (JP 55031463 A1). Here as well, the requirements are different in that both Ru and Pd do not have any effect in the Ostwald process and SiO2 is chemically unstable under the conditions of the Ostwald process. JP 06182203 relates to fluoride-containing carriers for noble-metal catalysts.
According to DE 40 20 914 A1, ammonia undergoes combustion almost without any formation of laughing gas if it is brought into contact with Pt, Pd, Rh or Ir, in combination with at least one of the oxides of Mo, V. The process described is, however, not used for large-size NH3 combustion, but for removing low concentrations of NH3 from waste gases.
According to DE 35 43 640, pure laughing gas at Pd can be decomposed properly, for example on corundum, alumina or silicic acid. However, palladium is completely inactive in case of a reduction of N2O in the Ostwald process.
For example, the following systems have become known specifically for use with the Ostwald process:
DE 198 19 882 A1 describes a catalyst for the decomposition of N2O, said catalyst being arranged downstream of the gauze catalyst and upstream of the heat exchanger and being provided as fixed-bed catalyst. In particular, it is a combination of CuO−Al2O3 with tin or lead.
DE 41 28 629 A1 discloses a silver catalyst with Al2O3 as carrier material.
DE 100 16 276 proposes CuO-containing catalysts. For example, a catalyst on CuO—Al2O3 base was tested within an industrial scope. The reduction of N2O achieved in a plant operated atmospheric pressure was 80 to 90% and that achieved in a medium-pressure plant (5.5 bar) at Antwerp was approx. 70% (G. Kuhn: Proceedings of the Krupp Uhde Technologies Users Group Meeting 2000, Vienna, 12-16 Mar. 2000). NO losses were specified to be <0.5%. The fact that copper might be dissolved out of the catalyst is pointed out in (Applied Catalysis B: Environmental 44 (2003) S.117-151). Since the decomposition of ammonium nitrate is catalysed by copper, this would be a serious safety problem.
According to US 2004023796, a catalyst for the decomposition of N2O at 250 to 1000° C. was developed on the base of Co-oxide spinels on a CeO2 carrier (CO3-xMxO4, wherein M Fe or Al and x are 0 to 2). NO losses were specified to be <0.2%. Similar cobalt-oxide-containing systems have already been recommended for the oxidation of ammonia (EP0946290B1).
U.S. Pat. No. 5,478,549 describes the utilization of ZrO2 as N2O decomposition catalyst. This teaching is further developed in WO 0051715 in that iron and, optionally, transitional metals can be admixed to the ZrO2 pellets during their production.
A mixed-oxide catalyst (ZrO2 and Al2O3) is the subject of WO 9964139. The catalyst which is impregnated with a zirconium salt is intended to partially convert N2O (approx. 15%) to NO. Altogether, the N2O is to be degraded by 78 to 99%. However, this requires that the catalyst be provided in very big amounts and, as a rule, that the converter be modified.
Most of these catalysts have deficiencies which may be related with technical safety—as with the above-mentioned catalyst on CuO—Al2O3 base—or these catalysts fail to be adequately stable under the conditions prevailing in the reactor. This applies both to the catalytically active component and the supporting structure which may, at the same time, have a stabilizing function as compared with the catalytically active component.
Moreover, there is a need of catalyst systems which efficiently remove N2O under the conditions of the Ostwald process.
Therein, the catalysts should meet the following requirements:                The decomposition of NO must not be catalysed because this would reduce the efficiency of the process.        If it is a fixed-bed catalyst, the catalyst concerned must have a very high activity and may cause only an extremely low pressure drop across the height of the catalyst bed. An increased loss of pressure may also cause a loss of efficiency during the production of HNO3, in particular if the existing technical plant does not allow any further increase in pressure.        The catalyst should not require redimensioning of the existing converters.        